Hitherto, as a method of analyzing a frequency of an acoustic signal, etc., generalized harmonic analysis has been used. In this method, the most dominant sine wave is extracted from the original time-series signal within an analysis region and, by using the residual components thereof as an input, the same process is repeated. Generalized harmonic analysis is described in “The Fourier integral and certain of its applications” by N. Weiner, Dover Publications, Inc., (1958).
According to this generalized harmonic analysis, since an influence of an analysis window (analysis region) is not imposed, accurate extraction of frequency components is possible with respect to a slight frequency variation of an input signal. Furthermore, the analysis region and the resolution of the frequency can be set independently of each other, and it is possible to predict a signal beyond the analysis region.
Therefore, as an apparatus for performing frequency analysis on a time-series signal such as an acoustic signal and for extracting specific frequency components, a frequency-component extraction apparatus using generalized harmonic analysis has been conceived.
FIG. 1 is a block diagram showing an example of the configuration of a conventional frequency-component extraction apparatus.
An input signal dividing section 11 divides, for example, an acoustic time-series signal into predetermined analysis regions when that signal is input as an input signal, and supplies the obtained input time-series signal to a frequency analysis section 12 and a subtraction unit 14.
The frequency analysis section 12 analyzes the input time-series signal by using generalized harmonic analysis, creates extracted waveform information, such as the amplitude and the phase, on main frequency components in an analysis region, and supplies the information to an extracted waveform synthesis section 13 and to, for example, a data compression section (not shown) provided outside a frequency-component extraction apparatus 1.
The extracted waveform synthesis section 13 performs predetermined waveform synthesis on the basis of a plurality of pieces of extracted waveform information supplied from the frequency analysis section 12, and outputs the obtained extracted waveform time-series signal to the subtraction unit 14.
The subtraction unit 14 performs subtraction in a time domain on the basis of the extracted waveform time-series signal supplied from the extracted waveform synthesis section 13 and the input time-series signal supplied from the input signal dividing section 11, and outputs the obtained residual time-series signal to an apparatus at a subsequent stage, provided outside the frequency-component extraction apparatus 1.
Next, the operation of the frequency-component extraction apparatus 1 of FIG. 1 is described with reference to the flowchart in FIG. 2. Each signal which is generated is described as appropriate using FIG. 3A. In FIG. 3A, an example of a signal in a case where there is no attack (sharp rise) or release (sharp fall) in an input time-series signal is shown.
In step S1, the input signal dividing section 11 divides an input acoustic time-series signal into predetermined analysis regions, and outputs the generated input time-series signal into the frequency analysis section 12 and the subtraction unit 14. For example, as shown in FIG. 3A, the input signal dividing section 11 divides an acoustic time-series signal at an analysis region L and outputs the resulting input time-series signal s1 to the frequency analysis section 12 and the subtraction unit 14.
In step S2, the frequency analysis section 12 receiving the input time-series signal computes frequency components at which the energy of a residual signal reaches a minimum when the frequency components are extracted from the input time-series signal. That is, in step S2, the frequency analysis section 12 computes the energy of the residual signal with respect to all the frequencies (frequency for each small region of a predetermined number of samples) of the analysis region in order to obtain the frequency at which the energy of the residual signal reaches a minimum.
In step S3, the frequency analysis section 12 subtracts a pure-tone signal corresponding to the frequency computed in step S2 from the input time-series signal in order to generate a residual signal. Then, in step S4, the frequency analysis section 12 creates extracted waveform information corresponding to the frequency computed in step S2 and supplies the information to the extracted waveform synthesis section 13. The extracted waveform information contains information, such as the frequency, the amplitude, and the phase, of the signal corresponding to the extracted frequency components. Furthermore, the frequency analysis section 12 outputs the extracted waveform information to an apparatus (not shown) provided outside the frequency-component extraction apparatus 1.
In step S5, the frequency analysis section 12 computes the energy (residual energy) of the residual signal generated in step S3, and determines whether or not the residual energy is less than a predetermined threshold value. When it is determined that the residual energy is greater than the predetermined threshold value, the process proceeds to step S6.
In step S6, the frequency analysis section 12 assumes the residual signal to be an input signal, and the process returns to step S2, where this and subsequent processes are repeatedly performed. That is, a plurality of pieces of extracted waveform information corresponding to the number of times in which the processes of steps S2 to S6 are repeated is supplied to the extracted waveform synthesis section 13.
When the frequency analysis section 12 determines in step S5 that the residual energy is less than the predetermined threshold value, the process proceeds to step S7.
In step S7, the extracted waveform synthesis section 13 performs predetermined waveform synthesis on the basis of the plurality of pieces of extracted waveform information supplied from the frequency analysis section 12 in order to generate an extracted waveform time-series signal. The extracted waveform synthesis section 13 generates, for example, an extracted waveform time-series signal s2 such as that shown in FIG. 3A. When the input time-series signal s1 does not contain an attack or release, the input time-series signal s1 and the extracted waveform time-series signal s2 become substantially the same waveform.
The extracted waveform time-series signal generated in step S7 is output to the subtraction unit 14. In step S8, a residual time-series signal is generated from the difference from the input time-series signal supplied from the input signal dividing section 11. That is, a residual time-series signal s3 becomes substantially a standing waveform, as shown in FIG. 3A, and in step S9, the signal is output to an apparatus (not shown) at a subsequent stage.
The extracted waveform information which is analyzed and output to a subsequent stage by the frequency analysis section 12 is coded and then stored or transmitted. Therefore, from the viewpoint of the amount of data, a lesser number of frequency components is preferable.
However, when the input time-series signal within the analysis region contains an attack or release, it is difficult to represent the attack or the release with a limited number of frequency components.
For example, as shown in FIG. 3B, when an input time-series signal s11 contains an attack or release, information capable of accurately representing the wave of the attack or the release cannot be supplied to the extracted waveform synthesis section 13. Consequently, in the residual time-series signal s13, components which do not originally exist appear before or after the portion where the attack or release has occurred, and the frequency components cannot be efficiently extracted.